FIG. 1 depicts a conventional power supply system 10 for an LCD panel. In the conventional system, the utility power, 110V/220V ac system is converted to a high-voltage dc either through a rectifier circuit or a power factor correction circuit 12. The high-voltage dc is then step down through a dc/dc converter 14 to provide low voltages, for example, 5V and 12V as the power sources for electronic devices such as micro-controller, memory, TFT driver, graphics and cold-cathode fluorescent lamps (CCFLs). The inverter 16 further converts the low-voltage dc to a high-voltage ac to provide power for the CCFLs in the LCD panel 18. Multiple power conversions through the DC/DC converter 14 and DC/AC inverter 16 impacts the conversion efficiency and also generates heat in the system. For large LCD panels such as for LCDTV applications, the majority of power consumption resides in CCFLs. Therefore, it is important to boost the efficiency of the inverter for the CCFLS.
FIG. 2 illustrates one conventional toplogy 20 that improves the efficiency of the inverter system. The high-voltage dc is directly applied to the DC/AC converter 16′. It eliminates an intermediate step of DC/DC converter and improves the overall efficiency.
Converting a high-voltage dc to an ac signal requires a lower turns-ratio transformer to the CCFL as illustrated in FIG. 3, FIG. 3A and FIG. 4. FIG. 3 depicts an inverter topology 30 that is built around a half bridge circuit (two switches) and includes an inverter controller 32 that drives two switches 34 and 36 to develop the necessary voltage across the transformer 38. The inverter controller 32 and half bridge topologies are well known in the art. FIG. 3A depicts a Class D inverter topology and FIG. 4 depicts a full bridge (four switch) inverter topology, as are well understood in the art. The topology also depicts feedback lines which are not important for this discussion. Since the size of the transformer is selected based on the CCFL applications, the secondary winding which drives the CCFL is generally fixed by the load requirements. However, the number of turns in the primary winding in high-voltage input application is much higher than the transformer being driven by the low-voltage input such as 5V to 20V DC, and this increases the complexity and cost of the transformer.